Two classes of reference circuits that are used in electronic circuits may be generally referred to as “reference voltage nodes” and “reference current sources.” A fixed-value reference voltage node can be created by using fixed-value discrete resistors configured in a resistive divider circuit. The accuracy of the reference voltage created by such a resistive divider circuit is determined by the accuracy of the fixed-value resistors and the accuracy of the voltage source(s) connected to the resistors. The accuracy of a fixed-value resistor is typically defined by a tolerance parameter that specifies the allowable variation of resistance from a nominal resistance value. For example, a 100 ohm, 10% resistor may be used in a circuit that permits a variation in resistance between 90 and 110 ohms, while a 100 ohm, 1% resistor may be used in a circuit that only permits a variation in resistance between 99 and 101 ohms.
The tolerance parameter of a resistor is controlled by several factors, one of which relates to a trimming procedure performed during manufacture of the resistor. The trimming procedure is used to remove excess resistive material from a resistor so as to produce a nominally accurate resistance value. Such a trimming procedure is applicable not only to discrete resistors but also to planar resistors such as those that are employed on a printed circuit board (PCB) or embedded inside integrated circuit (IC) packages. For example, the resistors of a reference voltage node wherein the node is a part of a larger circuit inside an IC, may be trimmed to obtain a desired reference voltage. Such trimming when carried out over a large number of ICs can become an expensive process, potentially resulting in the creation of an undesirable trade-off between circuit performance and cost.
In contrast to a fixed-value reference voltage node, a variable-value reference voltage node can be created by using a variable resistor that is also referred to as a potentiometer. A variable resistor permits a circuit-user the flexibility to vary the value of the variable resistor, thereby allowing programming of a reference voltage value based on specific requirements. Such requirements may be of a variable nature depending upon the needs of a multiplicity of customers or upon the multiplicity of needs of one customer, at any time subsequent to manufacture of the variable resistor. While user-controlled programming of a reference voltage node by the use of a potentiometer, provides an advantage in terms of flexibility-of-use, one shortcoming in doing so, relates to the possibility of accidental misadjustment of the potentiometer thereby leading to potential circuit malfunction.
Electronically-controlled potentiometers have been implemented inside ICs to a limited extent. But the use of such electronically-controlled potentiometers in conjunction with additional circuitry inside the same IC is relatively uncommon and may not be typically carried out in a cost-effective manner. For example, it is fairly untypical to provide an electronic potentiometer as a part of a variable-value reference voltage node, such a voltage node being in turn used in conjunction with a comparator circuit inside the same IC. As is known in the art, comparator circuits are used in many applications, including converter circuits such as analog-to-digital converters.
In addition to a reference voltage node, the second class of reference circuit used in various applications such as comparators and converters, is often referred to as a reference current source. A reference current source is typically created from a transistor circuit that incorporates one or more voltages and one or more resistors. The resistor values are selected either by selecting suitable fixed-value discrete resistors or by selecting suitable potentiometers, to generate appropriate currents in the transistor circuit. One example of a circuit used as a reference current source is known in the art as a current mirror circuit. The shortcomings related to resistors, described earlier with reference to voltage sources is also largely applicable to reference current sources.
Applications that use reference voltage nodes and reference current sources will be described in more detail using prior art figures. One such prior art figure, FIG. 1 illustrates an analog-to-digital converter (ADC). While such an ADC can be constructed using discrete devices, such as multiple voltage comparators and resistors that are placed upon a PCB, an ADC is often constructed using devices fabricated upon a substrate inside an IC. The IC packaging provides numerous benefits, yet suffers from the resistor-related handicaps outlined earlier. For example, the accuracy of each of reference voltage values Vref(n) through Vref(0) used in the ADC circuit, is dependent upon the accuracy of each of the resistors, thereby requiring a comparatively expensive trimming process during manufacture. Additionally, once the IC has been manufactured, the reference voltage values cannot be changed because the resistors cannot be readily modified to create other resistance values.
One example of a reason for desiring a change in reference voltage values may arise out of a change in user requirement that necessitates conversion of a linear ADC to a non-linear ADC. In one example of a prior-art linear ADC, each of the resistor values is selected to be identical, thereby creating a multiplicity of reference voltages such that the voltage difference between any one voltage reference node and its neighboring voltage reference node remains identical throughout the resistive divider chain. For example, if the difference between Vref(n) and Vref(n−1) is 0.5V, the difference between Vref(n−1) and Vref(n−2) will also be 0.5V.
On the other hand, in a non-linear ADC, each of the resistor values will be scaled suitably to produce a multiplicity of reference voltages such that the voltage difference between any one voltage reference node and its neighboring voltage reference node is different from a second voltage reference node and its neighboring voltage reference node. For example, if the difference between Vref(n) and Vref(n−1) is 0.5V, the difference between Vref(n−1) and Vref(n−2) may be set at 1.5V—a scaling factor of 3. Such a non-linear ADC will consequently require setting the values of the resistors to non-identical values.
FIG. 2 illustrates one exemplary embodiment of a prior art digital-to-analog converter (DAC) 200, which accepts a multi-bit digital input signal and produces an analog output voltage that reflects the state of the digital input signal. DAC 200 incorporates n binarily weighted current sources I0, I0/2, I0/21, I0/22, . . . I0/2n−1 that are switched to the output by n current switches located in current switcher 205. The most significant bit (MSB) of the digital input signal determines the state of the switch that switches the I0current, while the least significant bit (LSB) of the digital input signal determines the state of the switch that switches the I0/2n−1 current. When used as a current-multiplying DAC, it is common for a precision current mirror circuit 215 to generate reference current Iref, which is directly related to the output precision current MI0. The analog output current MI0is usually converted into the analog output voltage by an amplifier 220.
Current mirror 215 uses two transistors 216 and 217 that are connected to each other such that current Iref through transistor 216 is “mirrored” by current Iref through transistor 217. While FIG. 2 does not show resistors incorporated into the current mirror circuit 215, most practical applications utilize collector and/or emitter resistors that influence the value of the Iref current. The use of these resistors lead to the limitations described earlier, including limitations such as trimming costs and lack of user-programmability.
FIG. 3 show further details of current switcher 205 and the weighted current generator 210 of FIG. 2. Transistors 312 and 314 constitute one of several differential comparators inside current switcher 205. The LSB of the digital input signal controls the switching of a fractional value of the overall current (MI0) through transistor 312. The fractional value, which equals (I0/2n−1), is determined by the value of emitter resistor 316 inside the weighted current generator 210.
Transistors 322 and 318 constitute a second one of the several differential comparators inside current switcher 205. The MSB of the digital input signal controls the switching of a fractional value of the overall current (MI0) through transistor 318. The fractional value, which equals (I0), is determined by the value of emitter resistor 324 inside the weighted current generator 210. The sum total of currents that is produced at any instance by the various transistors that have been switched on by the corresponding bits of the digital input signal, constitutes the overall current (MI0) for any particular digital input signal.
Resistors 316 and 324 are part of a binarily weighted set of resistors, some of which are created by a multiplicity of resistors connected in parallel. The shortcomings of fixed as well as variable resistors that were described earlier, is applicable to this circuit also.
FIG. 4 illustrates a second exemplary embodiment of a prior art digital-to-analog converter (DAC) 400, which accepts a multi-bit digital input signal and produces an analog output voltage that reflects the state of the digital input signal. DAC 400 uses n binarily weighted current sources I0, I0/2, I0/21, I0/22, . . . I0/2n−1 that are switched to the output by n transistors 405, 410, . . . 415. The most significant bit (MSB) of the digital input signal determines the state of transistor 405 that switches the I0current, while the least significant bit (LSB) of the digital input signal determines the state of transistor 415 that switches the I0/2n−1 current. The analog output current MI0, which is the sum of the currents through transistors 405, 410, . . . and 415 for any particular digital input signal, is usually converted into the analog output voltage by an amplifier 420. Analog output current MI0is scaled to be proportional to Iref the reference current that is generated by a current mirror circuit (not shown).
Transistors 405, 410, and 415 constitute three of the n transistors used in DAC 400. These transistors are typically metal oxide semiconductor field-effect transistors (MOSFET). The value of the drain current through any one of these transistors is determined by the applied gate voltage and the source-gate-drain geometry of the device. One of the parameters that determine the relationship between gate voltage and drain current is termed the width/length (W/L) ratio of the channels that define the source, drain, and gate inside the MOSFET. Typically, if a certain drain current is obtained for a particular value of gate voltage, the drain current can be doubled with the gate voltage remaining unchanged, if the (W/L) ratio of the MOSFET is doubled.
As an example, transistor 410 is a MOSFET with a drain current of (I0/2) for a given gate voltage. The gate voltage in this case will be the digital bit that is one less than the MSB. Transistor 410 is shown in FIG. 4 as having a (W/L) ratio equal to (2n−1 (W/L)). Transistor 405 has a gate voltage which is identical to the gate voltage applied to transistor 410, because it is equal to a second digital bit (the MSB). Therefore to obtain a drain current in transistor 405 equal to (I0), which is double the drain current (I0/2) through transistor 410, transistor 405 is typically configured to have a (W/L) equal to (2n (W/L)). This (W/L) ratio of transistor 405 is twice the (W/L) ratio of transistor 410.
Such an exponential scaling of transistor sizes to accommodate a range of digital input signal values, is undesirable for several reasons. For example, the component area of the DAC circuit when implemented inside an IC, using a set of identical transistors would be much smaller than the component area when using a set of binarily-sized transistors. Apart from the sub-optimal use of the substrate, the performance of the DAC is also compromised due to several factors. One such factor is increased parasitics, which leads to limitations in sampling speed and bandwidth constraints. A second factor relates to matching the electrical operating characteristics of each transistor to the others in the set of transistors. Typically, the accuracy of a DAC such as DAC 400, will be determined by how well the transistors are matched to one another in providing an accurate binary current-scaling relationship.
Given the shortcomings of the prior art reference voltage nodes and reference current nodes used in various circuits such as analog-to-digital converters and digital-to-analog converters, it is desirable to provide alternative systems and methods that address such shortcomings.